1. Field of the Invention
The present invention relates to an image forming apparatus for recording on two sides in a single pass, with which a toner image can be formed on both sides of a recording medium in a single paper pass.
2. Description of the Related Art
Some known copiers, printers, facsimile devices, and other such image forming apparatus are designed with a two-side recording function, whereby images are formed on both sides of a recording medium such as paper. With such conventional image forming apparatus capable of recording on two sides, the standard approach is to employ a system whereby the image (visualized image such as toner image) from one side of an original formed on an image support such as a photosensitive member is first transferred to and fixed on the surface of paper, and then this paper is turned over by an inversion path or the like and fed through the machine again, so that the image (visualized image) on the other side of the original is transferred to and fixed on the back side of the paper. With this system, however, it is impossible to ensure adequate reliability of paper conveyance because the paper conveyance direction has to be switched, because paper curling occurs during the fixing of the image on side, and for other such reasons.
One known way to deal with this situation is to transfer and form toner images onto both sides of the paper in a single paper pass, that is, use an image forming apparatus capable of recording on two sides in a single pass. Such apparatus have been proposed, for example, in Japanese Patent No. 2,906,538 and Japanese Laid-Open Patent Application No. 2000-105513.
Nevertheless, with the image forming apparatus disclosed in the above-mentioned Japanese Patent No. 2,906,538, the polarity is switched for one of the two sides' worth of toner images formed on the photosensitive member (that is, enough for the front and back of the recording medium). This switching of the toner image polarity requires the provision of a polarity switching means such as an opposite-polarity corona charging means, which makes the structure that much more complicated. Also, with the image forming apparatus disclosed in Japanese Laid-Open Patent Application No. 2000-105513, two types of toner with different polarity are readied, the toner image on side is formed with one toner, and the toner image on the other side is formed with the other toner of different polarity. Consequently, this image forming apparatus requires two developing apparatus for holding two types of toner of different polarity, which again makes the structure that much more complicated.
In addition, Japanese Laid-Open Patent Application No. H1-209470 (pp. 2–3, FIG. 1) has disclosed an image forming apparatus having first and second image supports for forming toner images on the surfaces thereof, which is constituted such that the toner images formed on these first and second image supports are each transferred to the one or the other side of the recording medium to obtain recorded images. With this image forming apparatus, first and second transfer members are provided along a contact component where the first and second image supports come into contact, with the first transfer member being disposed further upstream in the movement direction of the first and second image supports than the second transfer member. The recording medium is fed in between the first and second image supports, which are in contact with each other, the toner image on the first image support is electrostatically transferred onto one side of the recording medium by the action of one of the first and second transfer members, and the toner image on the second image support is electrostatically transferred onto the other side of the recording medium by the action of the other transfer member. With a constitution such as this, the toner images can be transferred onto either side of the recording medium by passing the recording medium between the contact components of the first and second image supports.
With this image forming apparatus, however, there is the possibility that the toner image electrostatically transferred to one side or the other of the recording medium by the first transfer member will be electrostatically retransferred back to the image support side when the toner image is transferred to one side or the other of the recording medium by the second transfer member, and this can result in degradation of the quality of the recorded image formed on the recording medium.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Laid-open Patent Application No. H3-253881.